Denim is a rugged cotton twill textile, in which the weft passes under two (twi- "double") or more warp threads. This produces the familiar diagonal ribbing identifiable on the reverse of the fabric, which distinguishes denim from cotton duck. Denim has been in American usage since the late 18th century. The word comes from the name of a sturdy fabric called serge, originally made in Nîmes, France, by the André family. Originally called serge de Nîmes, the name was soon shortened to denim. Denim was traditionally colored blue with indigo dye to make blue "jeans", though "jean" then denoted a different, lighter cotton textile; the contemporary use of jean comes from the French word for Genoa, Italy (Gênes), where the first denim trousers were made.